It is sometimes necessary to reinforce composite structures, such as those used in aerospace industry in order to meet strength and/or stiffness requirements. These structures may be reinforced by adding stiffeners to the structure which may provide the structure with additional strength and rigidity. In the past, individual stiffeners have been attached to primary structural members using any of several secondary assembly processes, including but not limited to the use of discrete fasteners, bonding, co-bonding and co-curing. Each of these processes may have disadvantages, such as, without limitation, the additional time and labor to complete the assembly process and/or the need for relatively expensive capital equipment such as autoclaves used to co-cure parts. Additionally, in some cases, the stiffeners may comprise multiple parts which may add undesirable weight and/or part count to a vehicle and/or increase the time and labor required to complete the assembly process. In some applications where the stiffeners are integrated into the structural member, the ends of the stiffeners, referred to as “runouts”, may not blend or transition smoothly into the member, which may result in undesirable stress concentrations in the structural member. Existing runout designs have discontinuous fibers at trimmed edges that may only moderately reduce stress concentrations in the surrounding structure.
Accordingly, there is a need for a simple, cost effective method of making stiffened composite structures in which stiffeners are integrated into structural members to form a unitized structure. There is also a need for a stiffener design having runouts that blend smoothly into the structural member and reduce or eliminate stress concentrations at the ends of the stiffeners.